(1) Industrial Application Field
The present invention relates to an electronic air-fuel ratio control apparatus in an internal combustion engine, which is provided with an electronically controlled fuel-injecting apparatus and has a function of performing feedback control of the air-fuel ratio by controlling a fuel injection quantity based on a signal from an oxygen sensor arranged in the exhaust system of the engine.
(2) Related Arts
An electronically controlled fuel-injecting apparatus in an internal combustion engine has a fuel-injecting valve in the intake system of the engine to inject a fuel at a predetermined timing synchronously with the revolution of the engine or a predetermined time period. In this electronically controlled fuel-injecting apparatus, a basic fuel injection quantity is set based on parameters of driving states of the engine (such as the flow rate of air sucked in the engine and the revolution number of the engine etc.) participating in the quantity of air sucked in the engine. A final fuel injection quantity is determined by appropriately correcting the set basic fuel injection quantity.
According to one method for performing this correction, an oxygen sensor is arranged in the exhaust system of the engine, and the correction is performed based on a signal from the oxygen sensor under predetermined engine-driving conditions. More specifically, the air-fuel ratio of an air-fuel mixture sucked in the engine is detected through the oxygen concentration in the exhaust gas by this oxygen sensor, and the output voltage (electromotive force) abruptly changes with the point of combustion of the air-fuel mixture at the theoretical air-fuel ratio being as the boundary and a lean signal of a small output voltage or a rich signal of a large output voltage is emitted. Based on this lean or rich signal, an air-fuel ratio feedback correction coefficient is set by proportion-integration control, and a fuel injection quantity is computed by multiplying the basic fuel injection quantity by the air-fuel ratio feedback correction coefficient, whereby the air-fuel ratio is feedback-controlled to the theoretical air-fuel ratio.
Under driving conditions where the concentration of nitrogen oxides (hereinafter referred to as "NO.sub.x ") in the exhaust gas, exhaust gas recycle (EGR) control of reducing the NO.sub.x concentration by lowering the combustion temperature by recycling a part of the exhaust gas to sucked air is carried out in parallel to the above-mentioned air-fuel ratio control.
However, in the EGR system for reducing NO.sub.x, since an EGR passage or an EGR control valve is necessary, the structure is complicated and the cost is increased. Moreover, the combustion efficiency is drastically reduced by introduction of the exhaust gas into the mixture to be sucked into the engine and the output performance is degraded, and by lowering of the combustion temperature, the emission amounts of unburnt components such as CO and HC are increased.
Under this background, an oxygen sensor comprising an NO.sub.x -reducing catalyst layer for promoting the reaction of reducing NO.sub.x was proposed by the present applicant (see U.S. Pat. Application Nos. 117,507 and 117516).
The brief function of the NO.sub.x reducing oxygen sensor will now be described hereinafter. The conventional oxygen sensor emits a high or low voltage with respect to a certain slice level basing on an oxygen concentration in the exhaust gas from the engine and when the output voltage is reversed between the high and low voltage the air-fuel ratio is recognized as the theoretical air-fuel ratio. However, the conventional oxygen sensor can not detect the oxygen concentration in the NO.sub.x component in the exhaust gas which should be taken into consideration as a part of oxygen concentration in the exhaust gas since the oxygen component in the NO.sub.x might be used for the combustion of the fuel and therefore the oxygen component should concern the oxygen concentration in the air-fuel ratio. Therefore the theoretical air-fuel ratio detected by the conventional oxygen sensor has represented only the pretended theoretical air-fuel ratio which is richer than the real theoretical air-fuel ratio by the oxygen concentration including in the NO.sub.x. Further the pretended theoretical air-fuel ratio has changed in response to the concentration of the NO.sub.x which has been produced with the concentration changeable due to the various engine driving states. Such an unprecise detection of the theoretical air-fuel ratio has resulted in unprecisely controlling of the air-fuel ratio in the lean side of the true theoretical air-fuel ratio by the electronic air-fuel ratio control apparatus so that increasing of the NO.sub.x concentration was performed (FIG. 9) and that the inferior combustion of the mixture in the combustion chamber of the engine and consequently the inferior engine performance was carried out and also a conversion efficiency of the ternary catalyst mounted on the exhaust system was worsened in an emission condition (FIG. 10).
On the other hand the proposed NO.sub.x -reducing oxygen sensor can reduce NO.sub.x to detect the oxygen concentration in NO.sub.x with the result of the output value thereof in response to the real air-fuel ratio which is not influenced by the change of the NO.sub.x concentration.
A method proposed in U.S. patent application Ser. No. '88, 179,535. in which the air-fuel ratio feedback controls are performed by using the NO.sub.x reducing oxygen sensor to precisely and stably control the air-fuel ratio to the true theoretical air-fuel ratio richer than the pretended theoretical air-fuel ratio controlled by the conventional oxygen sensor, whereby the NO.sub.x conversion efficiency of the ternary catalyst for purging the exhaust gas, is improved to reduce NO.sub.x, and therefore omission of EGR becomes possible because of reduction of NO.sub.x.
In these controls, we examined the relation of the basic air-fuel ratio obtained from the fuel injection quantity computed without correction by the air-fuel ratio feedback correction coefficient to the concentrations of NO.sub.x, CO and HC, and the following results were obtained (see FIG. 11).
(1) When the basic air-fuel ratio which is initially set is rich, the effect of reducing NO.sub.x by the control using the oxygen sensor having the NO.sub.x -reducing catalyst layer is not attained and the levels of CO and HC are not changed but kept high.
(2) When the basic air-fuel ratio is rich, the NO.sub.x -reducing effect is high, and the levels of CO and HC are not changed but kept low.
(3) When the basic air-fuel ratio is appropriate, the NO.sub.x -reducing effect is moderate and also the levels of CO and HC are moderate.
Accordingly, it is at least necessary that the basic air-fuel ratio should not be rich.
Of course, no problem arises during the feedback control of the air-fuel ratio in the stationary state, but even during the feedback control of the air-fuel ratio, at the transient driving where the follow-up delay of the feedback control is caused or at the stoppage of the feedback control of the air-fuel ratio, the dependency on the basic air-fuel ratio increases and a problem arises.
The present invention is to solve this problem, and it is an object of the present invention to provide a system in which the above-mentioned oxygen sensor comprising an NO.sub.x -reducing catalyst layer is combined with an apparatus for learning and controlling the basic air-fuel ratio, and the feedback control of the air-fuel ratio is performed by the oxygen sensor while the basic air-fuel ratio is learned and controlled to an appropriate or lean level, whereby the efficiency of the purging the exhaust gas by a ternary catalyst can be highly improved without any influence by the deviation of the basic air-fuel ratio owing to unevenness of parts and the like.
For attaining the above-mentioned object, a first aspect of the present invention provides an air-fuel ratio control apparatus of an internal combustion engine, which comprises, as shown in FIG. 1, the following means (A) through (I) (first invention):
(A) an engine driving state-detecting means for detecting the driving state of the engine, including at least a parameter participating in the quantity of air sucked in the engine, (B) an oxygen sensor disposed in the exhaust system of the engine to detect the air-fuel ratio of an air-fuel mixture sucked in the engine through the oxygen concentration in the exhaust gas, said oxygen sensor comprising a nitrogen oxide-reducing catalyst layer for promoting the reaction of reducing nitrogen oxides and emitting a lean or rich signal with the point of the theoretical air-fuel ratio corresponding to the nitrogen oxide concentration in the exhaust gas being as the boundary, (C) a basic fuel injection quantity-setting means for setting a basic fuel injection quantity based on said parameter detected by the engine driving state-detecting means, (D) a rewritable learning correction coefficient-storing means for storing a learning correction coefficient for correcting the basic fuel injection quantity according to the engine driving state, (E) a learning correction coefficient-retrieving means for retrieving a corresponding learning correction coefficient of the engine driving state according to the actual driving state of the engine from the learning correction coefficient-storing means, (F) an air-fuel ratio feedback correction coefficient-setting means for increasing or decreasing by a predetermined quantity the air-fuel ratio feedback correction coefficient for correcting the basic fuel injection quantity according to the rich or lean signal from the oxygen sensor, (G) a fuel injection quantity-computing means for computing a fuel injection quantity based on the basic fuel injection quantity set by the basic fuel injection quantity-setting means, the learning correction coefficient retrieved by the learning correction coefficient-retrieving means and the air-fuel ratio feedback correction coefficient set by the air-fuel ratio feedback correction coefficient-setting means, (H) a fuel-injecting means for injecting and supplying a fuel to the engine in an on-off manner according to a driving pulse signal corresponding to the fuel injection quantity computed by the fuel injection quantity-computing means, and (I) a learning correction coefficient-renewing means for learning the deviation of the air-fuel ratio feedback correction coefficient from the reference value according to the engine driving state and rewriting the learning correction coefficient of the learning correction coefficient-storing means so as to reduce said deviation.
A second aspect of the present invention provides an air-fuel ratio control apparatus of an internal combustion engine, which comprises the following means (J) in addition to the above-mentioned means (A) through (I):
(J) a learning correction coefficient-shifting means for correcting the learning correction coefficient so as t shift the air-fuel ratio to the lean side.
In the present invention, the basic fuel injection quantity-setting means sets the basic fuel injection quantity based on parameters participating in the quantity of air sucked in the engine, which are detected by the engine driving state-detecting means. The learning correction coefficient-retrieving means retrieves a learning correction coefficient corresponding to the actual engine driving state from the learning correction coefficient-storing means. Furthermore, the air-fuel ratio feedback correction coefficient-setting means sets the air-fuel ratio feedback correction coefficient, by decrease or increase of a predetermined quantity, according to a lean or rich signal from the oxygen sensor having an NO.sub.x -reducing catalyst layer. The fuel injection quantity-computing means computes the fuel injection quantity by correcting the basic fuel injection quantity by the learning correction coefficient and also by the air-fuel ratio feedback correction coefficient. The fuel-injecting means is actuated by a driving pulse signal corresponding to the computed fuel injection quantity.
By the actions of the oxygen sensor and air-fuel ratio feedback correction coefficient-setting means, the feedback control of the air-fuel ratio is performed. Since the oxygen sensor has the NO.sub.x -reducing catalyst layer, when the NO.sub.x concentration in the exhaust gas is increasing, the NO.sub.x component is reduced by the oxygen sensor so as to detect the real oxygen concentration. The output voltage of the oxygen sensor abruptly changes when the air-fuel ratio detected by the sensor at the point slightly richer than the pretended theoretical air-fuel ratio which was detected by the no NO.sub.x -reducing oxygen sensor and a lean or rich signal is emitted with this point being as the boundary. Accordingly, if the feedback control of the air-fuel ratio is performed based on the detection result of this oxygen sensor, the air-fuel ratio is controlled to the true theoretical air-fuel ratio richer than the pretended theoretical ratio even when the NO.sub.x in the exhaust gas is changed in respect to various engine driving states and therefore decrease of NO.sub.x in the exhaust gas can be attained.
Separately, the learning correction coefficient-renewing means learns the deviation of the air-fuel ratio feedback correction coefficient from the reference value with respect to each area of the engine driving state and renews the data of the learning correction coefficient-storing means, corresponding to the area of the engine driving state, so as to reduce said deviation.
By this learning control, the basic air-fuel ratio is optimalized, and even at the stoppage of the air-fuel ratio feedback control or at the transient driving, the effect of reducing NO.sub.x can be attained.
If the learning correction coefficient-shifting means is used for slightly shifting the learning correction coefficient to shift the basic air-fuel ratio to the lean side as in the second aspect of the present invention, the effect of decreasing NO.sub.x is further improved and CO and HC can be controlled to lower levels.
The present invention will now be described in detail with reference to an optimum embodiment illustrated in the accompanying drawings, but the present invention is not limited by the embodiment and the present invention includes changes and modifications within the range of objects and technical scope of the present invention.